Cardiovascular disease is the leading cause of morbidity and mortality in western countries. Despite substantial medical progress in identifying modifiable cardiovascular risk factors, a significant proportion of individuals with cardiovascular disease have no known risk factors. Klotho is a recently discovered hormone found to protect against endothelial dysfunction, atherosclerosis, and cardiovascular disease in mice, as well as to extend lifespan. The klotho gene encodes a single-pass transmembrane protein that exists as a circulating hormone and as a membrane-bound co-receptor. Klotho regulates nitric oxide production, vascular permeability, suppression of inflammation, and expression of adhesion molecules by the endothelium. In the absence of klotho hormone, endothelial dysfunction and hypertension occur in animal models, which are reversible or preventable with in vivo delivery of the klotho gene. Until recently, there was no way to measure levels of klotho in the circulation. We are the first to measure klotho levels in a large number of people. Our preliminary data support the premise that klotho is protective against cardiovascular disease in humans. Based upon knowledge of klotho and our preliminary data, we hypothesize that older people with low circulating klotho levels are at increased risk of cardiovascular events and mortality, and that klotho levels are associated with impaired brachial flow-mediated dilation. We propose to characterize the relationship of serum klotho with prevalent cardiovascular disease (myocardial infarction, angina, stroke, congestive heart failure, peripheral artery disease, transient ischemic attack), carotid intima-media thickness, brachial flow-mediated dilation, cardiovascular events and mortality in the Cardiovascular Health Study, an NIH-supported, population-based, longitudinal study of coronary heart disease and stroke in US adults. We will also conduct a genome-wide association study of serum klotho levels. This project will determine whether serum klotho is a major risk factor for cardiovascular disease and will provide novel insight into the role of klotho in human health. The project should provide the rationale and evidence for klotho as a potential novel therapeutic target for intervention in cardiovascular disease.